The present invention relates to fuel pumps, particularly of the type for supplying fuel at high pressure for injection into an internal combustion engine.
Typical gasoline direct injection systems operate at substantially lower pressure level when compared, for example, direct injection diesel fuel injection systems. The amount of energy needed to actuate the high-pressure pump is insignificant in the total energy balance. However, in a system with a constant output pump and variable fuel demands all of the unused pressurized fuel has to be returned into the low-pressure circuit. A good portion of the energy originally used to pressurize the fuel is then converted into thermal energy and has to be dissipated. Even a relatively modest heat rejection (200-500 Watt) will result in fuel temperature increase (especially if the fuel tank is only partially full) and this will further worsen problems resulting from low vapor pressure of a typical gasoline fuel.
A variable output high-pressure supply pump would thus be very desirable. Furthermore, the speed range of typical gasoline engines is substantially wider than that of diesel engines (e.g., from 500 RPM at idle to 7000 RPM or higher at rated speed). With variable pumping pressure achieved, for example, with a demand controlled pump, it would be easier to optimize the injection rate at any engine speed.
Current mainstream demand control strategies use a fast solenoid controlled valve to spill fuel from the internal high-pressure circuit back into the pump sump during the time when no fuel addition into the rail is desired. The internal high-pressure circuit is separated from the rail by a no return check valve. As the volume of this circuit is relatively small, after initial pressure drop, the rest of the fuel quantity supplied by the pump is spilled at a relatively low pressure (if desired it can be as low as just above the feed pump pressure). Because of that the heat rejection of such a system is much lower, compared to a system constantly spilling pressurized fuel (i.e., constant output pump with spilling rail pressure regulator.) However during high-speed operation even this lower heat rejection might not be acceptable as it could cause excessive temperature increase.
Several other configurations for a demand-based direct injection gasoline supply pump are shown and described in U.S. patent application Ser. No. 09/342,566, filed Jun. 29, 2999 for xe2x80x9cSupply Pump For Gasoline Common Railxe2x80x9d, now U.S. Pat. No. 6,345,609, and International application PCT/US00/04096 published as WO/0049283, the disclosures of which are hereby incorporated by reference. The present invention can be considered as particularly well suited for implementation in one or more of the embodiments shown in these publications, as well as variations thereof. In particular, the present invention is an improvement to the variable output control concept described in said International publication, for further decreasing the unproductive heat energy to be rejected.
The invention can broadly be considered as a hybrid method for controlling a common rail gasoline fuel injection system having a high pressure supply pump to the common rail, wherein the improvement comprises the combination of low speed control by recirculating the excess pump discharge flow to the fuel tank or through the pump inlet at a pressure lower than the rail pressure, and high speed control by premetering or prespilling.
In the preferred embodiment, the unwanted fuel at high speed is spilled out of the pumping chambers, before the high pressure is generated in the first place. This not only has the benefit of reduced heat rejection, but the additional benefit of a gradual pressure increase during the spill valve closing. As a result, any vapor cavities created during the restricted charging will implode at a slow rate before the high pressure pumping starts, resulting in lower noise and less likelihood of cavitiation erosion. Also, the spill valve will be closing against gradually increasing pressure and by that it will be potentially faster, or else the same value speed can be realized with lower magnetic force. With the spill occurring only after the natural end of pumping, the duty cycle can be extended in order to be easily controllable, even at maximum speed. Furthermore, the valve opening speed is not relevant at high engine speed, as the pumping event already ended with the piston reaching top dead center (TDC). Thus, the valve can be optimized for the closing event by using a weaker return spring, or the magnetic force can be generally reduced, resulting in a smaller and less expensive solenoid valve and associated control circuit.
The invention may be better understood in the context of a gasoline fuel injection system for an internal combustion engine, having a plurality of injectors for delivering fuel to a respective plurality of engine cylinders and a common rail conduit in fluid communication with all the injectors for exposing all the injectors to the same supply of high pressure fuel. An electronic engine management unit includes means for actuating each injector individually at a selected different time, and for a prescribed interval, during each cycle of the engine. A high pressure fuel supply pump having a high pressure discharge passage is fluidly connected to the common rail, and to a low pressure feed fuel inlet passage. The method and associated system establish at least two control regimes corresponding to respective low and high engine speeds. During low speed operation, unregulated low pressure fuel is fed to the pumping pistons, and the common rail is intermittently isolated from the pump, such that during the isolation, fuel discharged from the pump is diverted to a location of relatively low pressure in the fuel supply system, upstream of the pump. During high speed operation, the quantity of low pressure fuel pressurized from the pumping pistons, is regulated, thereby reducing the quantity of highly pressurized fuel delivered to the common rail.
A first, low speed control subsystem controls the discharge pressure of the pump between injection events, by diverting the pump discharge so that instead of delivery to the common rail, the flow recirculates through the pump at a lower pressure. This is preferably accomplished by a recirculation control passage fluidly connected to the low pressure feed fuel inlet passage, a discharge control passage fluidly connected to the high pressure discharge passage, and a non-return check valve in the high pressure discharge passage, between the discharge control passage and the common rail, which opens toward the common rail. A control valve is fluidly connected to the recirculation control passage and to the discharge control passage, and switch means are coordinated with the means for actuating each injector, for operating the control valve between a substantially closed position for substantially isolating the recirculation control passage from the discharge control passage and a substantially open position for exposing the recirculation control passage to the discharge control passage.
A second, high speed control subsystem for regulating feed quantity can be implemented in a variety of ways including a calibrated orifice, a proportional solenoid valve, pre-spilling, or pre-metering. In the preferred embodiment, the same solenoid valve used for the intermittent diversion or recirculation of pump discharge at low pressure is utilized at a different point in the timing cycle, to effectuate pre-spill for the high speed control regime.
The invention may also be considered a method for controlling the operation of a high pressure common rail direct gasoline injection system for an internal combustion engine having a continuously operating high pressure fuel pump to receive feed fuel at a low pressure and discharge fuel at a high pressure to a check valve which opens to deliver high pressure fuel to the common rail. During low speed operation, after each injector actuation an hydraulic control circuit is opened upstream of the check valve, whereby the pump discharge passes through the control circuit instead of the check valve, at a decreased pressure from the high pressure to a holding pressure between the high pressure and the feed pressure. While the pump discharge passes through the control circuit but immediately before each injector actuation, the hydraulic circuit is substantially closed whereby the pump output pressure rises from the holding pressure to the high pressure. When the pump output pressure reaches the high pressure an injector is actuated. At high engine speed, one or more of the previously mentioned quantity regulating techniques is implemented for quantity control of the fuel that is actually pumped at high pressure.
The major advantages of this control strategy are the control simplicity and quiet operation (acoustic and hydraulic noise) as well as torque uniformity at low speeds, where the driver""s perception will be most sensitive.
It should be appreciated that the two control regimes may be distinct, i.e., the control passes from one regime to the other through a transition zone at a transition speed, or the control regimes may be super imposed, i.e., low pressure recycling of excess fuel may continue at higher speed after the transition speed is reached such that for at least some of the higher speed conditions, both low pressure recycling and regulated feed quantity to the pumping chambers occur simultaneously.